1. Field of the Invention
The present invention relates generally to radio-frequency (RF) and/or microwave components, and particularly to RF and/or microwave coupled transmission line components.
2. Technical Background
Couplers are four-port passive devices that are commonly employed in radio-frequency (RF) and microwave circuits and systems. A coupler may be implemented by disposing two conductors in relative proximity to each other such that an RF signal propagating along a main conductor is coupled to a secondary conductor. The RF signal is directed into a first port connected to the main conductor and power is transmitted to a second port disposed at the distal end of the main conductor. An electromagnetic field is coupled to the secondary conductor and the coupled RF signal is directed into a third port connected to the secondary conductor. The secondary conductor is connected to a fourth port, commonly referred to as the isolation port. The term isolation port refers to the fact that, ideally, the RF signal is not available at this port.
Those of ordinary skill in the art will understand that directional couplers operate in accordance with the principles of superposition and constructive/destructive interference of RF waves. When coupling occurs, the RF signal directed into the input port of coupler is split into two RF signals. At the isolation port, the two incident signal and the coupled signal are substantially out of phase with each other and cancel each other. In practice, the cancellation is not perfect and a residual signal may be detected. The residual signal, of course, is a measure of the performance of the device. The output signal at the port directly connected to the main transmission line, and the coupled output port, are substantially in phase with each other and constructively interfere, i.e., the incident signal and the coupled signal reinforce each other. It should also be mentioned that the coupled output signal is typically out of phase with the output of the main transmission line.
In any event, coupled transmission lines are commonly used in RF/microwave circuits and systems to achieve a variety of functions. Many of the applications may only require a 3 dB coupler. For example, 3 dB couplers are often used in power splitter or power combiner applications. On the other hand, some applications may specify 5, 6, 10 and 20 dB coupling as typical numbers. In other words, less than half the incident power is directed to the coupled port. For example, a coupler may be employed to sample an RF output signal for use by a power level monitor. For example, the power level monitor circuit may require the coupled port to provide a signal −20 dB down from the incident signal. Another example of asymmetric coupling is an attenuator application. Other coupler applications include, but are not limited to, return loss cancellation and/or improvement, balanced amplification, and balun implementation. A balun may be implemented, for example, as a Marchand balun, an inverted balun, a Guanella balun or a Ruthroff balun. In each of the aforementioned balun implementations, coupling plays a major role in determining the impedance transformation ratio. One unique aspect of balun design relates to the use of an “overcoupled” coupler in certain implementations. An overcoupled coupler is a coupler with more than half the power going to the coupled port.
Those of ordinary skill in the art will understand that device weight and volume are important issues for most implementations. A variety of approaches have been used to miniaturize couplers, such as meandered lines, spiral lines, lumped realizations, ferrite transformers and electrical short couplers. One drawback associated with meandered couplers relates to the fact that they experience even/odd mode phase velocity imbalance as the lines are meandered tighter and tighter. Because of the constructive/destructive interference properties described above, this imbalance tends to negatively impact coupler performance.
Conventional spiral design configurations have drawbacks as well. The phase angle from one turn to the next of a spiral must be small relative to the wavelength or this implementation will also experience even/odd mode phase velocity imbalances. Lumped discrete component implementations are limited because they support a very narrow signal bandwidth. Additional discrete components must be employed to provide a coupler having a sufficiently wide bandwidth.
While ferrite transformer type couplers have very wide bandwidth, it is difficult to achieve arbitrary coupling values with ferrite couplers. Further, ferrite transformer couplers are inherently bulky and labor intensive.
So called “electrical short” couplers employ a combination of lumped elements and coupled transmission lines. The transmission lines are typically less than a quarter wavelength (λ/4). As the length of the transmission lines in the implementation are shortened, the bandwidth decreases to that of a fully lumped component implementation.
In other approaches, coaxial and waveguide couplers have been considered for coupler implementations. However, these implementations are rarely used in high volume applications because they are relatively expensive to manufacture. Further, these designs are difficult to integrate into RF systems. Thus, these coupler types are impractical.
The most commonly used couplers are referred to as the broadside coupler, edge coupler and the interdigital edge coupled design. The interdigital edge coupled transmission lines are commonly known as Lange couplers. To achieve high coupling in edge coupled transmission lines, the spacing between the coupled lines must be small. This spacing is determined by the capabilities of the photolithographic patterning process. Because of these manufacturing difficulties, it is difficult to produce 3 dB couplers using this method. In fact, coupling values do not typically exceed 10 dB.
Broadside couplers refer to the fact that the wide portion of the TEM transmission lines are disposed in the coupler facing each other. The broadside coupler includes two transmission lines separated by a homogeneous dielectric material. The transmission lines are interposed between two outer ground planes. Dielectric material is likewise disposed between each ground plane and the adjacent transmission line. This configuration supports TEM propagation and, unlike the microstrip interdigital couplers, even and odd mode phase velocities are equal. This results in relatively good bandwidth, directivity, and VSWR. Furthermore, broadside couplers may be used to implement 3 dB couplers. However, those of ordinary skill in the art will understand that transmission line spacing must be relatively small or the line widths must be wide, or both.
What is needed is a broadside coupler implementation that may be configured to achieve any desired coupling value without the constraints experienced by the conventional devices described above. Further, a coupler implementation is needed that may be implemented within in a desired form factor for a given performance specification.